This invention relates generally to birefringent crystals, and more particularly, to a temperature-stabilized birefringent crystal for use in stable, temperature-independent birefringent crystal interferometers.
Interferometers form the basis of a number of important communications devices including interleavers, dispersion compensators, and periodic filters. The basic function of an interferometer is to split coherent light into two paths with possibly different propagation delays and then to recombine the light from these two paths.
If the coherence length of the light source is longer than the difference in the path lengths on the two arms, interference of the two optical signals at the output provides a sensitive measure of the difference in the propagation delays on the two arms. If the frequency of the input light source is swept, the interferometer reveals a periodic transmission with a frequency period of                               v          o                =                  c                                                    n                1                            ⁢                              l                1                                      -                                          n                2                            ⁢                              l                2                                                                        (        1        )            
where c is the speed of light in free space, ni and li are the optical index and physical path length of the two arms i=1,2. This quantity is typically referred to as the interferometer free spectral range (FSR).
To provide stable operation, both nili products must be stable to much better than one wavelength of light. At optical communications wavelengths, this leads to a stability requirement on the order of 10 nm, which is difficult to maintain. It is known in the art that a birefringent crystal interferometer (BCI) can be designed to provide such stable path lengths.
In BCIs comprising uniaxial birefringent crystalline material, the two xe2x80x9cpathsxe2x80x9d are simply the optical paths of the two orthogonal linear polarizations propagating through the material. Since one polarisation experiences the extraordinary refractive index, ne, and the other the ordinary refractive index, no, the path difference, also known as the retardance, is given by xcex94nl, where the birefringence, xcex94n, is given by nexe2x88x92no for a material having positive dielectric anisotropy. The interferometer FSR is now given by:                               v          o                =                  c                      Δ            ⁢                          xe2x80x83                        ⁢            nl                                              (        2        )            
Since l1=l2= automatically, only changes in the birefringence, xcex94n, and the total crystal length, l, can affect the operation of the interferometer. In fact, changes in temperature modify both of these properties, to an extent characterised by the physical parameters of thetno-optic coefficients (in fact, the difference of the two thermo-optic coefficients relating to no and ne) and the coefficient of thermal expansion (CTE) in the direction of light propagation.
One solution to the temperature dependence has been proposed by Kuochou Tai et al. in copending application No. 09/476,034, to the same assignee. Kuochou Tai et al. teach cascading two crystals of different material in such a way that the two crystal BCI is independent of temperature. For example, an appropriately selected Yttrium ortho-Vanadate (YVO4) crystal followed by an appropriately selected rutile (TiO2) crystal provides an interferometer that is stable to both thermal and mechanical perturbations.
However, the proposed method has several disadvantages.
First, it is difficult to manufacture both crystal lengths with sufficient accuracy. To overcome this limitation, pairs of crystals are picked so that the combination has the
Second, although this pair of crystals is selected to have the desired FSR and to be temperature stable, they typically do not resonate precisely at the optical frequency of interest (typically a channel on the ITU grid). To adjust the resonant frequency of the interferometer, the current practice is to add a thin piece of quartz (typically in the range of 180 to 210 microns). Accordingly, each crystal pair must be measured and an appropriate quartz piece selected to adjust the resonant frequency.
Finally, if during mounting, one of the crystals is rotated relative to the other, it is possible for the effective length and/or the effective indices of refraction (and hence effective birefringence) to change, introducing undesirable changes into the FSR and its temperature dependence.
It is an object of this invention to provide a birefringent crystal whose retardance is independent of temperature, for use in a thermally stable BCI that obviates the disadvantages of prior art.
It is a further object of this invention is to provide an optical retardance system that can be used in a stable, temperature independent BCI for use in interleavers, periodic filters, and/ or dispersion compensators.
The instant invention relates to a stable, temperature-insensitive birefringent crystal interferometer (BCI) that uses a single variety of crystal and that can be used in an interleaver, a periodic filter, and/or a dispersion compensator. In comparison to BCIs that use two crystal varieties, the resulting device is cheaper, more robust, and has better performance.
In accordance with the invention there is provided an optical system comprising: a first block of light transmissive uniaxial birefringent material having an input port and an output port, the material having a first retardance at a first temperature; and, straining means for inducing a strain in one of the first block and a second block of light transmissive material optically coupled to the first block, the strain induced for maintaining a second net retardance substantially unchanged from the first retardance at at least a second other temperature.
In accordance with the invention there is further provided a method for compensating a thermal drift of a birefringent material comprising the steps of: providing a first block of light transmissive birefringent material having a first retardance at a first temperature; and, maintaining a net retardance substantially equal to the first retardance at a second temperature by applying a stress to one of the first block of light transmissive birefringent material and a second block of light transmissive material.
In accordance with the invention there is further provided a method for compensating a thermal drift of a birefringent material comprising the steps of: providing a light transmissive element having a net retardance at a first temperature, the light transmissive element comprising a block of the birefringent material; and, maintaining the net retardance of the light transmissive element at a second other temperature by inducing a strain in at least part of the light transmissive element.
Conveniently, the term xe2x80x9cnet retardancexe2x80x9d as used herein, refers to the net or total retardance in the spectral region of desired device operation.